Because T cell activation is a highly regulated event with crucial consequences to the host, it is important to understand mechanisms that promote as well as down-regulate it. We discovered DC-HIL, a type I transmembrane protein expressed constitutively at high levels by epidermal Langerhans cells and dendritic cells (DC). Soluble DC-HIL bound to activated (but not naive) T cells, indicating that expression of its T cell ligand (DC-HIL-L) requires activation. In T cell activation assays, immobilized DC-HIL acted as a negative agonist, markedly attenuating T cell proliferation and IL-2 secretion triggered by signaling via the T cell receptor. By contrast, soluble DC-HIL augmented the mixed lymphocyte reaction and exacerbated contact hypersensitivity (CH) when injected intraperitoneally into mice during hapten-challenge (but not during hapten-priming); herein, DC-HIL acted as an antagonist interfering with binding of DC-HIL to DC-HIL-L. We hypothesize that the DC-HIL/DC-HIL-L pathway transmits a potent inhibitory signal to activated T cells and plays an important role in down-regulating effector T cell responses. Our specific aims are to: (1) Determine whether DC-HIL expression on DC modulates antigen presenting cell function in vitro. We will examine the ability of DC engineered genetically to over-express or silence DC-HIL, to activate T cells. (2) Characterize the inhibitory function of DC-HIL/DC-HIL-L pathway in vivo using the CHmodel. We will: identify DC-HIL- or DC-HIL-L-expressing cells in skin and lymph nodes (LN) of hapten-sensitized mice; characterize LN T cells in sensitized mice treated with soluble DC-HIL; compare DC-HIL/DC-HIL-L and PD-1/PD-L pathways for potency, synergy, and kinetics of down-regulated T cell activation; and examine DC-HIL knockout mice for phenotypic and functional changes including CH responses. (3) Identify DC-HIL-L on activated T cells by biochemical methods and an expression cloning strategy. Our results are likely to show great promise for DC-HIL and DC-HIL-L as potential therapeutic targets for immunologic or pharmacologic modulation.